hacker-pschorr

Good point Mark, if anonymousagain's 928 had a 3.09 rear end he would have walked away from the NSX

mark kibort

so, what drag can 1 more square foot of frontal area cost ? (if the drag coef. is near the same?)

again, rounding error at best.

Ill be at the track with a couple this weekend. Ill do a frontal picture of both to see the diff. from the same distance.

Mk

Jim bailey - 928 International

The NSX is 71 inch by 46 or 3266 the 928 is 51.5 x 72.28 or 3708 about 442 sq inches....divide that by 144 sq inch and you get about THREE SQUARE FEET of delta. Now multiply by the drag coefficient .... and that my friends is HUGE !! especially at speeds over 100 mph

mark kibort

only in first gear (if he could have traction) , as 4th with the 3.09 would have been near 4.5:1, which is the same as 3rd in the S4 or

mk

mark kibort

13% greater drag if the drag coeficients are equal, right?
so, thats the same as if the car went from .34 (stock S4 ) to .38. (same as increasing drag coefficient by 13%)

again, dirtying up the aero of a 928 , like mine, witih a big wing, drives the drag up in a big way, but that big way, as i mentioned, in hard accurate numbers, might be near 20lbs of drag. again, 20lbs of drag is the same effect as a little over 4ft-lbs of torque at the rear wheels. If this is a huge difference, then yes its huge. I can tell you from experience, that my straight line speed has not been too effecte by my change of wing settigns at any part of the track. again, all rounding errors.

I see same HP vipers and GT3RS cars (same RWHP in the area of 440rwhp) and both cars at high speeds, still accelerate near the same. you want to see big difference in aero, look at a viper frontal area vs a 996!

at a track like road america when top speeds are in the 150 to 170mph range, yes, this can be the differnce, but for the street or tracks like laguna or sears, that are still pretty fast tracks, its not going to make a measureable difference in performance. Put it this way, i would have no problem in racing someone with similar rwhp and a car that was half the size but the same weight . oh, wait, i do this every month. Take a look at the S2000 specs or the frontal area of the M3 e36. Now the S2000 has 330rwhp and weighs only 2400lbs, but when were near the same HP to weight, we were pretty dead even. (even though he was as much smaller in frontal area as the NSX will be.)

"HUGE" drag, is really only effecting fuel efficiency, for the reasons stated above. When it takes 20hp to keep a car rolling at 55mph, if the HP requirements go up by 5hp, then, you just lost 25% fuel efficiency! Thats when drag is "Huge".

Mk

mark kibort

The spec says 19.2 square feet for the NSX and 21.09 for the 928.

Looks like a difference of only 10%

again, you can look at the drag coef and multiply the frontal area and get a comparitive number for the relationship of the two cars as far as drag forces.

since the drag coeff are both near the same, in the .32 to .34 range, the CdF^2 number will be near 10% off. again, its the same thing if one car is .33 and the other is .36. insignificant as far as accelerative forces at 100mph and lower, but as far as MPG, yes, that is HUGE!

MK

Here is a simple formula to see how "huge" the effect for drag as it relates to HP:

Aero Horsepower = 0.000007 x Frontal Area x Cd x Speed3

Here is a Chart from Sport Compact Car

Car Frontal Area Cd HP Needed at 100 mph HP Needed at 125 mph HP Needed at 150 mph
2000 Subaru Impreza RS 22 0.35 54 105 182
2000 Subaru Impreza L 21 0.32
1998 Ford Mustang GT 22.5 0.34 54 105 181
1996 Honda Civic DX Hatchback 20.5 0.32 46 90 155
2001 Volkswagen Golf GL 20.5 0.31 44 87 150
1997 Acura NSX 19.2 0.32 43 84 145
2000 Honda Insight 20.4 .25 36 70 120

Jim bailey - 928 International

Just for grins...F-1 Car .... "The deceleration force under braking is usually 4 g (40 m/s²), and can be as high as 5 g when braking from extreme speeds, for instance at the Gilles Villenueve circuit. Here the aerodynamic drag actually helps, and can contribute as much as 1.0 g of braking force, which is the equivalent of the brakes on most sports cars. In other words, if the throttle is let go, the F1 car will slow down under drag at the same rate as most sports cars do with braking, at least at speeds above 150 km/h. ..." Mark your "wing" may have little effect on drag because it has little downforce and is mounted in turbulent air. NASCAR has determined that opening the lid above the oil tank makes a car FASTER

icsmike

After trying to read and understand this page all I can do is **** myself and make gurgle noises.

Jim bailey - 928 International

Says that all you need to go 100 MPH is 45-55 HP depending on the car. Sure makes doing a 100 seem pretty insignificant...and that a Honda Insite will go 150 MPH !!

BC

I continue to say it, and after this post, I will not argue if you go into a gearing charade () but, the S2000 is not a very aerodynamic car. It has a huge hole in the front of the car, and the top dimensions couldn't be good for aero.

Also, you have way (way?) more torque, and thats why you are even, even though you are heavier than him. Because his engine has such a short period of peak power, and subjectively speaking, such a short people of *any* power, you have him on equal terms with your higher weight, but technically longer time on power band.

mark kibort

Jim,

quit fighting the obvious. you bring up an F1 car's decelleration from "EXTREME" speeds. do you know what the drag coefficient of a F1 car is? you might want to post it. keep in mind, it is able to put down enough down force to drive upside down. say thats 2000lbs, if so, then at best case, thats 200lbs of drag. (probably much worse) so, that could be near 200hp at 100mph. there is part of your answer. at 200mph, have any idea what the forces would be?? (just for grins)

My wing is not in turbulent air. it is in some deflected air and puts down near 250lbs of downforce. (pretty substantial by any standards) but then again, why wouldnt it, its in a fairly clean air stream, and it is a well designed porsche cup car wing) Its probably more effective grabbing air that has been deflected .
In the end, regardess, it does produce a lot of downforce and subsequentially, create enough drag to more than equal the difference of a 10% greater or lower Cd. My point was to show how insignificant it was for our cars at speeds at or around 100mph and slower. 4ftlbs of engine torque, if we are talking 200lbs of downforce, which surely would make a 928 in the .39 to .42Cd range.

again, the difference in Cd is proportional to the diff in drag or frontal area. for gas mileage, its a HUGE deal, for acceleration, its a rounding error at best. when comparing our family of street, race cars. not F1

mk

PS you can see from the aero data, that my wing is CLEARLY in the yellow area.


we are talking about 100mph and less. we

mark kibort

i had a '73 that went 120mph and it probably had less than 100hp. and at 100mph, i was no where near full throttle. my wifes civic had 80rwhp and it went 100mph easy!! most all do!
Thats pretty accurate, at least in the ball park. The point was at 50mph, how much power do you need?
if the answer is 25hp or so, then if you have 20lbs of drag at 100mph, that would be near double the power required. would kind of KILL fuel efficiency. But, if you are racing someone with or without the wing or drag, its the equivilant of 4ft-lbs of engine torque. Make sense?? im not making this stuff up, i swear. its basic physics 101
Im even speaking in terms of force or torque now, you should be happy

mk

mark kibort

Brendan, you are now guessing and making things up.

I have seen the S2000 torque HP curve. he also has a close ratio gear box, and is able to be in the max hp range for as long as I am. Actually, now with his valve fixed, (was at 310rwhp) at 320rwhp, he has more hp and has more area under the HP curve for the operating range. all you have to do is watch the video to see. his advantage is mostly in HP to weight.

Now, im not going into gearing other than if a peakier HP curve has closer gears the net time spent in the HP range will be the same .

NOW, more torque??? after all i have discussed with the list, you dare bring this up again?? Bendan, acceleration at ANY SAME SPEED, is proportioal to HP or rear wheel forces (or torque if the tires are the same diameter) , not engine torque .

remember :
acceleration = power/(mass x velocity) ????????

so , at any same vehicle speed, if we are at the same HP we will both put down the same torque at the rear tires, regardless of engine torque!!!!!!
this is EXTREMELY easy to prove. and yes, now not only do we have the same rear wheel HP at any speed, the S2000 has 300lbs less weight.
I can live with his cornering and braking advantages, but not the acceleration advantages. Just too much to overcome!

Mk

BTW the S2000 starts off a little better than our cars with aero Cd.
Here is a pic of the s2000. i have as big a hole under the nose as the next guy

edit: also if that hole in the nose is vented properly, it can be much less drag and effective frontal area than if it was closed off. which, both the S2000 and my 928 are.

Jim bailey - 928 International

Mark is is far from obvious ....MPG is a direct measure of how much ENERGY it requires to travel at a given speed to say that it is otherwise seems odd to me Let's see from the table it takes about twice the power to go 125 mph than it does 100 mph and about three times as much to go 150 MPH. The F-1 article stated at speed over 120 Kph that is about 90 MPH. But you are correct at the low speeds at which you spend much of your track time the effect of drag and frontal area is less important When you try to accelerate from 100 MPH to 150 mph your rate of acceleration slows as more and more of the available horsepower/torque is being wasted consumed by the work of moving through the air. Your car at 170 MPH or so no longer accelerates. That is one reason that racing gear boxes tend to have smaller RPM drops in the higher gears.

mark kibort

you just restated much of the obvious. if 10% or 15% increased Cd or frontal area would equal that same increase in drag, and it was near 4ft-lbs of torque to drive the extra drag, and you had 20hp or so to cruise at 50mph. your rate of doing work (ie HP) or HP -seconds (a unit measure of energy) would increase proportionally. This means that you could reduce your MPG by the additional drag to a tune of a 25% reduction. hmmmm, no wonder the H2 Hummer gets 10mpg .

The rate of acceleration is greatly dependant on the aero factors from 100mph to 150mph, but what we are talking about is the net change. if for example that same 200lbs of down force on a wing, causing a .40 Cd, then that 200lbs at 150mph would then grow to 450lbs . the drag would have grown from 20lbs to 45lbs. (and the gear ratio would be 3.2 vs the 4.5:1 in 3rd at 100mph)
SO, in the end, the 45lbs of drag costs 14 ft-lbs of engine torque. (through a 3.2:1 ratio at 6000rpm at 150mph) Is this huge???? well,it certainly is more of a factor now, but the equivilant hp is near 14hp as well at that speed, so is that a huge difference??? I think a 300lb car weight advantage is a huge advantage. as far as HP equivilance, its more like 30hp, or double the discussed (HUGE) differences.

Mk

edit: Dont forget the acceleration goes down propotioal with speed, even if here wasnt any rolling friction and aero . ( newton identity for constant Power, best case) yes, aero losses magnify the losses in acceleration.
We need to look at the differential losses for two cars with differnt drag figures. the difference is not that great , even up to 150mph. the range is 4ft-lbs to 15ft-lbs for a clean car and a dirty aero car, like a race car. (ie .32 vs .40+)